Within satellite telecommunication systems using simultaneously SS-TDMA (satellite-switched time-division multiple access) on-board multiple beam antennas, incoming traffic switching from each of the communication receiving beams onto each of the communications transmitting beams, is achieved through a switching matrix, whose task is to determine, in a cyclic manner, the origin-destination interconnections required by the time plan, generally synthesized at the ground station starting from the traffic requirement and its evolution, and sent to the satellite via telecommand.
In telecommunication systems which adopt SS-TDMA and multibeam antennas as well as demodulation, regeneration and demodulation techniques on board, together with digital modulation of signals, the switching matrix acts directly upon demodulated and on board regenerated binary data.
In this case, switching must be such that the significant part of the transient message through the satellite is not disturbed: in particular, state transitions within the switching matrix must take place during "guard times" between ground station emission bursts. To optimize the frame filling efficiency it is convenient to keep guard times short; therefore there must be a mutual time synchronization between the on board clock, which sets the matrix switching operation, and burst emissions toward the satellite by the ground stations.
If besides traffic switching operations on satellite, also processing of digital signals takes place, such as, for example, decoding, reformatting, change of signalling or data rate change operations, it is almost indispensable to perform these operations at the same speed for all data streams arriving to the satellite and a speed which is as close as possible to the signalling speed (symbol speed) of the digital streams arriving at the satellite.
Due to ground oscillator instability and Doppler effect induced frequency shifts in the ground to satellite path, due to satellite residual movement around its station point, data streams arriving to the satellite are asynchronous (in frequency and phase). It is therefore necessary to perform a resynchronization of these streams before proceeding with further processing.
In on board switching systems which also require fast time plan reconfiguration, or origin to destination interconnections from frame to frame, to cope with demand assignment of satellite capacity, it is necessary to send to the satellite a number of compounds for the dynamic reconfiguration of the switching matrix on the order of several thousands per second. This signalling requirement largely exceeds the satellite telecommand capability which, at the very best, is of the order of a few hundreds of bits per second. Therefore it is necessary to provide alternative systems to effect this information data transfer between ground and satellite.
Below I will discuss some of the known solutions for bit and pattern synchronization.
These two main problems (bit synchronization and pattern synchronization), arising from differing operational requirements, may be solved simultaneously if we can find a way to deliver to the satellite a very stable signal which is close (or identical) to the digital stream symbol frequencies sent to the satellite, which simultaneously:
(a) synchronizes all asynchronous data streams arriving at the satellite; and PA1 (b) acts as a reference frequency signal to be used in the switching matrix timing circuits through successive divisions. PA1 (a) a groundbased measurement and control system for the on board reference generator, of the direct or indirect type (satellite transmitted symbol frequency measurement); PA1 (b) the use of operational procedures to process this information and the preparation of telecommands to be sent to the satellite; PA1 (c) the frequent use of the telecommand channel; PA1 (d) the on board use of additional circuitry for active control of oscillating frequency after telecommand reception and decoding, with negative impact on reliability. PA1 reacquire the reference signal frequency; PA1 shift to the reference frequency track mode; PA1 perform frequency measurement during the remaining part of the burst; PA1 compare it with the measurement taken during the corresponding burst in the previous frame; and PA1 generate a correction signal, if required, to set the frequency of the reference oscillator. PA1 firstly it depends on the number of stations active and operating within the antenna beamwidth considered; and PA1 secondly it depends on the degree of activation (burst emission) of the communication stations. PA1 (1) The condition that the ground terminal population within the area covered by the antenna beam is limited can often occur. PA1 (2) The station activating factor may vary considerably depending on the type of services offered, in particular in presence of telephone and nontelephone mixed traffic.
At least three techniques are known through which the above may be developed. However they all have significant drawbacks.
A short description of these known techniques follows, to highlight the advantages which are offered by the method of this invention.
(1) Stable autonomous source method.
Frequency drifts are controlled through telecommands. This method relies upon the use, on board, of a very stable frequency source usually based upon a temperature compensated quartz oscillator.
To minimize frequency drifts due to temperature fluctuations on board, a very sophisticated proportional thermostatic control of the oscillator is required, which implies considerable mass and power onsumption. Both of these factors are detrimental on board as satellite. Moreover, even with the very best proportional thermostats, one cannot achieve the desirable limits for frequency drift, which must be less than 10.sup.-8 as explained below. A further negative feature is crystal ageing, which may imply large frequency drifts, which are also seldom predictable.
Consequently fine frequency control system of the reference generator has to be adopted, relying upon telecommand.
Unfortunately, the fine frequency control telecommands of the reference generator must be sent to the satellite at a rate which is greater to the extent that the oscillator temperature control system is "looser".
Accordingly, if from a power consumption and weight viewpoint one tried to simplify the oscillator temperature control, this would require recourse to frequent telecommands to maintain its frequency within the required limits around the nominal values.
To make frequent recourse to telecommands is also a negative factor from an operational viewpoint, as it implies:
(2) Extraction of signalling frequency from the bursts delivered by one of the network stations.
This method implies the adoption of one of the burst emissions sent to the satellite in TDMA (time division multiple access) as a reference for system synchronization.
The system is locked onto the signalling (symbol) frequency of the data stream transmitted, i.e. by one of the ground stations.
As communications station emissions consist of bursts, on board there is no time continuity of the reference signal.
Therefore the on board reference signal recovery system is basically a time sampled phase locked device.
In other words this circuit, for each useful burst reaching the satellite, must:
During the time between two successive bursts, one can adopt a memory device which will avoid sudden frequency shifts when the phase locked loop is opened without the input signal being present.
The memory device time constant must be much larger than the time between bursts if we want to avoid that the oscillator drifts excessively from the desired lock frequency (the oscillator will attempt to work asymptotically around its natural frequency, which differs from the reference signal frequency, due to age or temperature effects).
Unfortunately, a very long time constant which is required for open loop operation, does not tally with the need for wide band during reacquisition and closed loop frequency tracking, i.e. in the presence of an input signal when the new burst arrives.
Therefore the circuit must include a device which inserts or removes the memory circuit depending upon whether the input signal, onto which the phase locked circuit must lock, is present; or, in an equivalent manner, the phase locked circuit must switch bandwidth depending on the presence or absence of the signal.
There are some substantial problems in the practical development of such a system in an operational situation.
The first is related to the detection of the presence of the signal to be locked before switching bandwidth (excluding the memory). Unless there is a way, on board, to know a priori which time window containing the master station burst will appear at the circuit input, it will be necessary to add further circuitry to detect signal presence and to consequently switch the loop bandwidth, with the consequent circuit complexities, reliability reduction, weight and power consumption increase.
A second aspect is related to input signal and digital demodulator threshold phenomena which act upon the signal before it reaches the circuit.
To understand this aspect, we must remember that a system which has to lock onto the data stream symbol frequency of a station assumes that we are working upon the data stream output by a digital demodulator.
Independently of the latter's specific construction, we can state--without losing general applicability--that the demodulator works correctly only when the modulated carrier to thermal noise ratio (C/No) is greater than a preestablished threshold.
Correct operation is when the demodulator provides a data flow from which one can still extract information tied to the input signal symbol frequency. Only under this condition can the reference signal recovery circuit--which operates on the residue recognizable component, at the arriving signal symbol frequency--operate correctly with a jitter which depends on the signal-to-noise ratio established at the demodulator output.
Below the demodulator threshold, there is no assurance that the demodulator will provide a significant output to recover the reference signal.
It can be easily understood how in the presence of large fading effects (i.e. as when carrier frequencies in the 20/30 GHz bands are used) due to atmospheric conditions, the demodulator may be working below threshold at the same time the bursts are delivered by the ground station.
The system would lose synchronization, since master station signalling frequency lock-on cannot be achieved.
3. Lock onto a signalling frequency.
Another synchronization system locks onto a signalling frequency which corresponds to the average of the signalling frequencies of more burst emitting stations, contained within one of the communication antenna beams.
An improvement to the method above is to perform lock-on so as to use all bursts, sent by all stations within the area covered by one of the communication antenna beams, which are demodulated, in time sequence by one demodulator.
The average of this method is that it is extremely unlikely that all stations will be simultaneously affected by anomalous propagation effects, implying carrier-to-noise ratios below the demodulator operational threshold.
Therefore, on the average, within the TDMA frame raster there will be many bursts having very good C/N ratios. These bursts will have differing signalling frequencies (due to the above mentioned Doppler effects and to the inevitable digital flow signalling frequency differences) but, in a plesiochronous network, such differences are generally very small.
Therefore an average signalling frequency and an average C/No ratio greater than the demodulator threshold value can be defined.
The phase locked loop circuit, under such conditions, will work better because the ratio between useful input signal presence to absence is almost unity.
It follows that the phase locked circuit does not necessarily need auxiliary circuits for frequency reacquisition and bandwidth switching, because it can be designated to track the average signalling frequency defined in the presence of C/No goog ratios. Unfortunately, the major drawbacks of this solution are that:
System effectiveness in fact increases with the number of active stations, while, in the extreme case, it may coincide with that of the system presented in paragraph 2 (supra) when only one stations is present within the area covered by the beam.
Furthermore it can easily be understood how the system may not provide adequate performance unless the number of stations is in excess of 3 to 5, also considering that the TDMA frame filling factor cannot be unity. In other words, traffic stations may not transmit bursts so as to fill the frame completely. Consequently this system may be interesting only when there are many stations per antenna beam or, in the case of few (3 to 5) stations per beam, when the average station activation factor which implies a TDMA frame filling factor is close to unity.
These two considerations are really much more important than might seem to be the case at first sight:
To say the least, this is the situation during the first years of system in operation, when the number of stations put into service increases gradually from a small number of prototypes to a preseries batch.
System operation must be demonstrated from the first phases of the exercize. Therefore a system which to operate assume that steady state will be reached asymptotically is neither optimum nor practical.
It may in fact happen that the greater part of stations usually adopted to sort telephone traffic, are muted to give way to a different type of communication via satellite (such as high definition TV program or teleconferences). Band occupancy, which is equivalent--in time--to the emission of very long bursts by part or by all stations affected by this type of communication, brings the system back to a situation with one or two stations per antenna beam, therefore equivalent to the situation described under consideration 2.